Demagnetization



April 11, 1939. AV 2,154,399

DEMAGNETIZATION Filed March 11, 1935 DEM/1 GNET/Z/IVG L. O. SOURCE ROTARY 6/4 17 1! Patented Apr. 11, 1939 mesa-menus Charles W, Davis, Pittsburgh, Pa.,- assignor to Chicago Development Company, Chicago, Ill.

Application March 11, 1985, Serial No. 10,513

12Clalms.

This invention is concerned with a method and with means for accomplishing the demagnetization of term-magnetic materials and substances containing such materials in a novel and more expedient, as well as more emcient manner than was heretofore possible. The principle of my invention may be successfully applied in all such cases. where demagnetization isdesired or required, for example, for the removal of permanent magnetism from minerals in the process of magnetic separation of ores, or for the demagnetization of magnetic materials for testing .purposes or in industrial processes requiring the demagnetization of materials preparatory to their use in electrical equipment and the like.

Demagnetization has been accomplished in the past by subjecting the magnetized material to successive fields of gradually diminishing strength and changed polarity. For example, if it is desired to demagnetize a rod-like magnetic body or the like, this may be accomplished according to orthodox methods by introducing the body into a solenoid the field of which is stronger than the field to which the body was previously subjected. The current is then decreased to zero and is subsequently reversed and increased again, applying, however, less field strength than previously. This procedure is repeated until a field force is finally applied which is practically zero.'

The body is then for practical purposes sumciently demagnetized. Alternating current of low or moderate frequency may be applied in the case of demagnetizing relatively thin bodies, applying in this case fields of slowly diminishing strength. Eddy currents apparently prevent the penetration of the magnetic effect to the core of bodies of larger size (skin or umbrella effect) and slowly diminishing direct current must be used. The initial strength of the field is preferably chosen high, particularly in the case of materials with unknown magnetic characteristics andespecially in the case of short and thick bodies because the demagnetizing effect willwbe most pronounced at the ends of such bodies. Special requirements may sometimes call for the use of a specially constructed apparatus for obtaining the proper current reversals and the uniform decrease of the 'field required for accomplishing the demagnetization, but ordinary, well-known equipment will in most cases sufllce.

Summarizing the above, it may be said that demagnetization has been accomplished in the past by subjecting the ferro-magnetic material to a magnetic field of greater strength than the coercive force of the material to be demagnetized and then reversing and simultaneously lowering the field in successive steps or stages until it approaches zero. The reversal of the current fiow and therefore of the polarity 5 of the magnetic field may be accomplished by means of suitable switching means or by using alternating current of the usual moderate frequency, that is, about 25 to 60 cycles. The lowering of the field strength may be accomplished 10 by removing the material gradually from the field or decreasing the current fiow. It was believed and repeatedly stated in the past that slow reversals of the field are preferable and also that slow reduction of the field to zero pro- 15 duced the best results. This is indicated by the insistence of prior investigators with reference to the uniform application of gradually diminishing field strength in a series of gradual successive current reversals, as briefly outlined go result is due to the fact that the speed of re- 30 versals which I employ is greater than that to which the poles of the magnetic substances can conform. In my method, the magnetic poles .are not able to orient themselves in the least favorable position for demagnetization but are acted 35 upon in a more favorable position and are thus demagnetized at a lower field strength.

The principal aim and object of my invention resides therefore in furnishing a new method and a new arrangement of apparatus for de- 0 magnetizing materials wherein the material is subjected to rapid reversals of current and rapid reduction of the field strength This may be accomplished in numerous ways, for example,

by subjecting the magnetic material to the action of high frequency oscillations, e. g., to the action of a damped oscillatory discharge in the field of a solenoid or the like.

In one embodiment of the invention, the material to be demagnetized is placed in a solenoid which is operatively connected in a circuit in which damped high frequency electric oscillations are generated. The oscillations decrease in strength from an amperage necessary to produce a magnetic field strength in the solenoid which is above the eiiective magnetic coercive iorce oi the material, to zero in a small iraction oi a second. It is diificult to determine the exact peak amperage oi such damped oscillations and I am thereiore unable to state just how much oi the advantage gained by my invention is due to the iact that the efiective coercive iorce oi the substance is less in a field activated with high irequency oscillations than in a slowly reversing field and how much is due to a peak amperage generated in this manner'being higher than could be generated in a circuit employing slow reversals oi the same current source. However, it is not necessary to evaluate these iactors ior the purpose at practicing my invention.

I shall now describe the manner in which my invention may be carried out in practice.

In the drawing. 1' 8- 1 illustrates one iorm oi circuit and associated apparatus with which my invention may be practiced, and Pig. 2 is a similar showing, modified with respect to the type oi switch employed and the arrangement oi the demagnetizing coil.

As shown in schematic iorm in Figure l oi the drawing, an ordinary direct current source, for example a 1l0-volt dynamo, may be employed in one embodiment oi my invention; with a suitable solenoid connected to the current source ior receiving the material to be treated. A suitable switch, for example a rotaryswitch, arranged to. make and break the circuit several times-per second, may be used and operatively connected in the circuit, and a condenser may be shunted across the switch. In this nt an oscillatory discharge will be set up in the circuit and will flow through the solenoid every time the circuit is broken by the switch. The ohmic resistance of the circuit will damp the current fiow to zero in a fraction of a second. The inductance and the capacity in the circuit are preierably adjusted and related to the operation oi the switch in such a manner that are formation is avoided upon breaking the circuit. The iormation oi a spark will not produce adverse eiiects. Otherwise the adjustment oi the various elements with relation to each other is not particularly critical. The values of some oi the elements in a practical arrangement, and the results obtained thereby are briefly noted below.

Favorable results were obtained in a circuit employing an ordinary current source including a 1l0-volt direct current generator and a solenoid connected thereto having 4000 turns oi No. 22 D. C. C. wire wound on a core three inches long and one and three-eights inches in diameter. A 4MMF condenser was bridged across the interrupter switch used in this circuit ior producing the oscillatory discharges. I have demagnetized with this arrangement cobalt steel oi 250 oersteds coercive force and reduced hematite with 360 oersteds coercive force. It should be noted that the number of turns in the solenoid should be related to the coercivity oi the material to be demagnetized, but the exact relation between these two iactors must be determined by experiment ior reasons explained previously. It is of no importance for the successful application of my invention whether the material is powdered or compact or whether it remains in the field or passes rapidly through it, provided, however, that it is subjected in its passage through the activated field to at least one complete oscillatory discharge. I have obtained excellent mults by employing fieldreversalsoithe order oimore than500 reversals per second.

Another embodiment oi my invention to be describedbelowisparticularlyadaptediorthe par.- poee oi carrying out the magnetic separation oi oresorthelikeinamoreefiicientmannerthan was proposed in the past.

Intheknown'artthe entrainmentoinonmagnetic particles has been obtained by activatingthe separating magnetwithinterrupteddirect current whereby the attracted mass started to iall awayiromthemagnetbutwas reattracted beiore it leit the field oi the magnet. Substantially the same eiiect was obtained with low irequency alternatingfieldsoi variouskindsarrangedtoproduce a winnowing action to disperse the particles. However, none oi these methods iurnishes either a complete and satisiactory demagnetization oi the magnetic particles or satisiactory and complete eiiect in avoiding entrainment oi the nonmagnetic particles.

These shortcomings are overcome by the above intimated embodiment oi my invention wherein I make use oi the demagnetizing eiiect oi high frequency currents by superposing them on an interrupted direct current circuit provided ior the activation oi a magnetic separator. Batisiactory demagnetization is accomplished thereby and entrainment is reduced to practically zero.

The corresponding arrangement, as shown in schematic iorm in Figure 2 oi the drawing, includes an interrupted direct current circuit in which a condenser oi suitable capacity is shunted across the interrupter switch. When the current is interrupted the magnetic mass tends to iall away irom the pole and is simultaneously sub- Jected to an oscillatory field which demagnetius it, causing the non-magnetic particles to be ireed. When the current is again established, the magnetic particles are reattracted but the non-magnetic particles continue to iall away irom the magnet.

I wish to mention in conclusion that damped oscillatory discharges may be produced in many known ways and most oi the known methods may be used in practicing my invention. A iamillar example is the mercury gap oscillator used with coreless induction iurnaces. The field produced in such a furnace is suitable for praticing my invention. However, due to the superposition oi several oscillations it is advisable to pass the material to be demagnetized rapidly through the activated field, ior example, the solenoid oi the iurnace, rather than allowing it to remain in the solenoid and interrupting the current flow.

While reierences to the method contained hereinabove are clear and understandable in principle to those skilled in the art, reierence may be had to the drawing ior a iurther consideration oi actual circuits and arrangements of parts which may be utilized.

Referring to Fig. l, Ishow conductors II and l I leading irom a direct current source, such as irom a direct current generator or commercial line carrying direct current. Where alternating current only is available, a suitable converter may also be used. The current is delivered to a demagnetizing coil i2, and the delivery thereoi is controlled by a suitably operated switch it. Any usual means, not shown, may be employed to control the periods at which this switch is operated. A condenser II is shunted acres the switch terminals, this condenser being of the general type described hereinabove, suitably balanced, oi course, with the circuit as a whole. In the iorm oitheinventionshcwninl'ig. Lthematerialto arses bemagnetisedmaybepassedthroughthed netising coil.

In Fig. 2, conductors it and it lead to a similar source of direct current and, through rotary switch l1," deliver current to a demagnetizing coil II. In this case. a core ll is provided and the material to be demagnetized, indicated at 21, is adapted to pass near one of the poles of the electromagnet which comprises the cell It and maining portion of the rotary switch functioning as a conductor to close the circuit. The rotary switch is operated at relatively high speed by suitable means not shown.

Where the method of my invention is employed in combination with magnetic separators, any of the usual direct current separators now on the market may'be employed. As an example, the separator described in U. 8. Patent No. 6,121 of February 20, 1849, and other patents issued from time to time utilizing the same generalprlnciples. may be cited. Since my inventionis not concerned primarily with magnetic separation but rather with the demagnetization of magnetic materials, whether associated with a process involving magnetic separation or not, I deem it unnecessary to disclose any type of magnetic separator in detail. since those skilled in the art are fully conversant with the features of magnetic separators and many difi'erent types are commercially manufactured and offered to the trade.

I have described certain embodiments of my invention and also specific examples of how and where it may be successfully applied in order to teach others the use thereof. It is understood, however, that the invention is capable of changes and modifications over and above the embodiments and uses explained and such changes and modifications may be carried out within the limit and scope of the appended claims. I have defined in these claims what I consider new and desire to have protected by Letters Patent of the United States.

I claim:

1. The method of demagnetizing ferro-magnetic material which consists in subjecting said material to a magnetic field generated in a solenoid by a damped high frequency oscillatory discharge.

2. The method of demagnetising ferro-magnetic material which consists in subjecting said material to a magnetic field generated in a solenoid by a series of damped high frequency oscillatory discharges.

8. The method of demagnetising substances whichconsistsinsublectingsaidsubstancesto thesctiono'fadevieeactivatedbydampedhigh frequency electric oscillations.

4. The method of demagnetising substances which consists in subjecting said substances to a magnetic field, reversing said field mccessively more than 500 times per second, and gradually reducing said field to zero. I

5. In a device for demagnetizing magnetic material, a source of direct current, electromagnetic means connected in circuit therewith for receiving said materials, and means for producing damped high frequency electric oscillations in said circuit.

6. In a device for demagnetislng magnetic material, a source of direct current, electromagnetic means connected in circuit therewith for treating said material, and means for producing damped high frequency electric oscillations in said circuit, said means including an interrupting device and a capacity cooperating therewith.

7. In a device for demagnetizing magnetic material. a source of current, electromagnetic means connected in circuit therewith for treating said materials, and means for producing damped high frequency electric oscillations in said circuit, said netic field whereby magnetic material contained in said substances is alternately strongly attracted by a magnet and allowed to fall away therefrom and demagnetizing the material during the period of less attraction by the action of a damped high frequency oscillatory magnetic field.

10. The method of demasneflzing relatively finely divided ferro-magnetic material, which comprises subjecting said material to a magnetic field generated in a solenoid by a damped high frequency oscillatory discharge.

11. The method of demagnetizing relatively finely divided ferro-magnetic material which comprises subjecting said material to a magnetic field. reversing the field successively more than five hundred times per second, and gradually reducing the field to sero.

12. The method of demagnetirrlng relatively finely divided ferro-mag'hetio material which comprises generating a magnetic field in a solenoid. reversing said field successively more than five hundred times per second, and continuously passingsaidfinalb'dividedmaterial intoandout of said field.

CHAR-LIB W. DAVIS. 

